Heretofore, the extrusion process for manufacturing resin-covered electric wires or the like has usually been performed by the cross head method. This prior art technique will now be described with reference to the drawings.
FIG. 7 is a sectional view showing a prior art crosshead apparatus. Referring to FIG. 7, designated at 12 is an extruder or the like for extruding resin 13, at 15 a die for molding the resin into a predetermined shape, at 14 a nipple for guiding the resin, at 16 a mandrel for leading a supplied wire, at 17 an adjustment nut, and at 18 a keyway (or a main fluid path).
The operation of the above prior art apparatus will now be described.
Resin extruded from the extruder 12 is forced out through the keyway 18, around the nipple 14 and through the die 15. Meanwhile, a wire 2 is supplied from a wire inlet and passes through the center of the nipple inside into the die 15. The resin 13 is molded around the wire 2 to produce a wire 1 which is covered with resin. The covered wire 1 is rolled up outside the die 15. When silicone rubber is used, the covered wire emerging from the die 1 is vulcanized in a tunnel oven or furnace (not shown) or the like before being rolled up.
The prior art nipple 14 for manufacturing electric wires is merely a hollow one through which wires can be passed. For manufacturing a catheter, a nipple as shown in FIGS. 8 and 9 is used.
Referring to FIGS. 8 and 9, the nipple 14 has diametrically opposed small bores 19 for supplying electric wires 2 therethrough and a central small bore 20. Through the central small bore 20, air or like gas is supplied under slight pressure into the central portion of the catheter to prevent squeezing of the central portion.
Heretofore, wire-enclosed cords such as wire-enclosed tubes, medical catheters and small diameter electric wires have not been in high demand so that those cords have not been required to be very fine, and thus they can be sufficiently formed with the nipple as shown in FIGS. 8 and 9. However, recently developed very fine wire-enclosed cords such as wire-enclosed tubes, medical catheters and small diameter electric wires cannot be manufactured using the prior art nipple. This is problematic for the following reasons.
(1) Thin metal wires can protrude out of the resin portion of the cord.
(2) The provision of an independent bore for supplying a thin metal wire therethrough requires sophisticated and difficult processing.
(3) In the prior art, it is possible to process a bore to a depth (or length) of about 3 to 5 times as large as the fore diameter the bore diameter. This means that a tip portion of the mandrel is deformed due to the resin extruding force.
(4) Wear due to passage of fine metal wire is significant, and early replacement of the nipple is necessary.
For the above reasons, very fine wire-enclosed cords such as wire-enclosed tubes, medical catheters and electric wires could have not been obtained by the prior art method.